1. Field of the Invention
This invention relates to a gas refining system for removing hydrogen sulfide contained in high-temperature and high-pressure reducing gases such as gas produced in a coal gasification process. More particularly, it relates to a gas refining system wherein either α-gypsum hemihydrate or gypsum dihydrate can be selectively formed as a by-product and, moreover, the reactor for forming such gypsum can be reduced in size or simplified.
2. Description of the Related Art
In recent years, the diversification of fuels (or raw materials) is advocated because of the exhaustion and rising cost of petroleum resources, and the development of techniques for utilizing coal and heavy oils (e.g., tar sand oil, shale oil, Taching heavy oil, Maya crude oil and vacuum distillation residue) has been promoted. As an example, attention has been paid to techniques for gasifying coal and heavy oils to utilize them for electric power generation and to produce fuels and raw materials for use in syntheses.
However, such gases obtained by the gasification of coal and heavy oils contain several hundred to several thousand parts per million of hydrogen sulfide, which must be removed for the purpose of preventing environmental pollution or protecting downstream equipment (e.g., gas turbines) against corrosion. Among the processes for removing hydrogen sulfide are dry processes which are advantageous from the viewpoint of thermal economy and simple in construction. One conventionally known example thereof is the dry gas refining process described in Japanese Patent Provisional Publication Nos. 63-123801 and 1-254226.
This gas refining process uses an oxide of a metal (e.g., Fe) as an adsorbent, sulfur compounds contained in a gas are adsorbed and removed by the adsorbent in the form of a sulfide, the adsorbent with reduced adsorption capacity is regenerated by roasting it with an oxygen-containing gas, and the regeneration gas containing sulfur dioxide formed by the roasting reaction is introduced into a reactor. In this reactor, using a gas blowing means, the regeneration gas and an oxygen-containing gas (usually comprising air) are blown into a calcium compound-containing slurry fed to the reactor, and thereby brought into gas-liquid contact with the slurry to effect the absorption of sulfur dioxide and the precipitation of gypsum within the reactor. In this process, α-gypsum hemihydrate is formed as a by-product by maintaining the temperature of the slurry within the reactor at 120-180° C.
As described in the above-referenced patent publications, the reactor used in this gas refining process consists of a pressure vessel which permits the regeneration gas and air for oxidization to be blown into the reactor under an elevated pressure (e.g., of 10 kg/cm2) and brought into gas-liquid contact with the calcium compound-containing slurry. As a gas blowing means, a so-called rotary atomizer has usually been employed.
This rotary atomizer comprises a hollow rotating shaft extending through the bottom of the reactor, and a gas is injected from the upper end of this hollow rotating shaft while it is rotated. However, since the injected gas bubbles have a relatively large diameter and its agitating action is weak, the gas bubbles are distributed only in a relatively limited area around the injection orifice provided at the upper end of the hollow rotating shaft.
As described in the above-referenced patent publications, the conventional gas refining process is designed so that only α-gypsum hemihydrate is formed as a by-product. This involves the following problems.
(1) Although α-gypsum hemihydrate (CaSO4.1/2H2O) has a relatively high commercial value in itself, the supply of water of crystallization converts it to gypsum dihydrate (CaSO4.2H2O). Accordingly, α-gypsum hemihydrate is sensitive to moisture, cannot be stored outdoors, and must be stored and conveyed under protection from moisture. Thus, its handling is very troublesome.
(2) In addition to the problem of protection from moisture which makes it difficult to store α-gypsum hemihydrate for a long period of time, the market (or demand) for α-gypsum hemihydrate is rather limited. According to the conditions of the market, therefore, it may be difficult to recover a high profit. Especially when the above-described gas refining process has come to be employed on a full scale in such facilities as electric power plants, there is a possibility that its supply will become excessive and its commercial value will decline steadily.
(3) On the other hand, gypsum dihydrate generally has a relatively low commercial value at present. However, it has sufficient characteristics for use, for example, as a raw material for the manufacture of cement which has a large market. Moreover, it can be stored outdoors and handled easily. Consequently, if it is possible to switch the by-product over to gypsum dehydrate according to circumstances and produce α-gypsum hemihydrate only when there is an active demand in the market, this will be advantageous from the viewpoint of profit recovery and other factors. However, this cannot be easily realized in the conventional gas refining process.
Moreover, in the above-described conventional gas refining process, the elevated pressure of the gases blown into the reactor (i.e., the regeneration gas and air for oxidization) reduces gas volume. In principle, therefore, the size of the reactor can be markedly reduced (especially in diameter) as compared with the case where the gas-liquid contact is effected at atmospheric pressure. This is highly advantageous, for example, in that a reduction in floor space can be achieved.
However, the use of a rotary atomizer as the gas blowing means makes it difficult to blow the regeneration gas and air for oxidization in the form of fine bubbles and distribute them uniformly throughout the internal space of the reactor. Eventually, the advantage brought about by blowing the gases under an elevated pressure (i.e., a reduction in size of the reactor) cannot be fully realized. Moreover, this also has the disadvantage of complicating the structure of the reactor.
As described above, a rotary atomizer has the disadvantage that, since the injected gas bubbles have a relatively large diameter and its scattering action is weak, the resulting contact efficiency is low. Accordingly, even if the gas bubbles can be distributed to the same extent in the lateral direction, the absorption and oxidation reactions do not proceed satisfactorily. As a result, it has been necessary to secure a large effective volume for gas-liquid contact, for example, by significantly increasing the vertical dimension of the reactor (or by considerably elevating the surface level of the slurry).
Moreover, as described above, the rotary atomizer causes gas bubbles to be distributed only in a relatively limited area around the injection orifice (having a maximum diameter of about 30 cm for practical purposes) provided at the upper end of the hollow rotating shaft. Accordingly, if it is desired to have gas bubbles distributed throughout the internal space of a reactor having an internal diameter, for example, of about 3 meters, it is necessary to install a plurality of rotary atomizer in side-by-side relationship at the bottom of the reactor. This has the disadvantage of complicating the structure of the reactor and causing an increase in cost.
Especially in the conventional construction where the motor of the rotary atomizer is disposed outside the reactor and its rotating shaft extends through the wall of the reactor, a special seal capable of withstanding the pressure difference between the inside and outside of the reactor needs to be used at that part of the wall through which the rotating shaft extends. This is very disadvantageous from a practical point of view.